we are trying to figure out if an hardware based VXLAN fabric (Cisco Nexus 9332PQ as spines and 9372PX as leaves) could be a plus for a small/medium datacenter with up to 20 hosts and 500 VMs or would be enough a "simpler" software VXLAN fabric using NSX-v ?

Well host count or VM count is not the criteria for Hardware based VXLAN . If there is a potential bridge use case - Software/Hardware based bridging is the choice . I have provided some inputs in this thread -Hardware Base VXLAN . Feel free to update if you have a different use case .